At temperatures greater than about 1000.degree. C. (1832.degree. F.), high temperature oxidation is the most important form of environmental attack observed with aluminide diffusion coatings. High temperature oxidation is a chemical reaction whose rate controlling process for an aluminide coating is the diffusion of aluminum through a product (oxide) layer. Diffusion is a thermally activated process, and consequently, the diffusion coefficients are exponential functions of temperature. Since the oxidation of aluminide coatings is a diffusion controlled reaction and diffusion coefficients are exponential functions of temperature, the oxidation rate is also an exponential function of temperature. At low temperatures where diffusion coefficients are relatively small, the growth rate of a protective scale on any aluminide coating is also small. Thus, adequate oxidation resistance should be provided by any state of the art aluminide coatings, such as: chromium aluminide, aluminide or two phase [PtAl.sub.2 +(Ni,Pt)Al] platinum aluminide, all inward grown coatings made by pack cementation. However, at high temperatures where the diffusion coefficients and consequently the oxidation rate increase rapidly with increasing temperature, only coatings which form high purity alumina (Al.sub.2 O.sub.3) scales are likely to provide adequate resistance to environmental degradation. This point is clearly illustrated by the cyclic oxidation test results from three state of the art aluminide coatings (designated MDC-51, MDC-351 and LDC-2E by the Assignee, Howmet Corporation) on IN-100 substrates provided in FIG. 1. The observed variations in life of coatings with the same initial thickness is the result of differences in the growth rate and/or adherence of the protective oxide scale. Specifically, the dissolution of substitutional substrate alloying elements into the aluminide coating and consequently the alumina scale can result in doping effects which can produce significant increases in the growth rate of the oxide scale. In addition, the presence of surface active tramp substrate impurities (S, P etc.) in the aluminide coating can have a detrimental effect on the adherence of the protective oxide scale. Thus, the inward grown chromium modified and simple aluminides (which contain all the elements in the substrate) exhibit poor resistance to high temperature oxidation. The presence of platinum in nickel aluminide has been concluded to provide a number of thermodynamic and kinetic effects which promote the formation of a slow growing, high purity protective alumina scale. Consequently, the high temperature oxidation resistance of LDC-2E coating is about five times better than the other coatings, see FIG. 1, in spite of the fact it is an inward grown coating.
In recent years, several limitations of the industry standard, two phase [PtAl.sub.2 +(Ni,Pt)Al], inward grown platinum aluminide coatings have been identified. First, the two phase coatings have metastable phase assemblages and thicknesses, as demonstrated in engine tests at both General Electric and Rolls-Royce. Second, the two phase coatings are sensitive to TMF (thermal mechanical fatigue) cracking in engine service, and the growth of these coatings in service only makes this problem worse. Third, the thick, inward grown platinum aluminides exhibit rumpling during both cyclic oxidation and engine testing. This phenomenon can have undesirable consequences when platinum aluminide coatings are used as the bond coat in thermal barrier coating systems. Fourth, the two phase platinum aluminide coatings are hard and brittle, and this can result in chipping problems during post coat handling and assembly operations.
Many of the problems encountered with the previous industry standard platinum aluminides can be attributed to the two phase, inward grown structure and can be overcome by using outwardly grown, single phase platinum aluminde coatings as described, for example, in the Conner et al. technical articles entitled "Evaluation of Simple Aluminide and Platinum Modified Aluminide Coatings on High Pressure Turbine Blades after Factory Engine testing", Proc. AMSE Int. Conf. of Gas Turbines and Aero Engine Congress Jun. 3-6, 1991 and Jun. 1-4, 1992. For example, the outwardly grown, single phase coating microstructure on directionally solidified (DS) Hf-bearing nickel base superalloy substrates was relatively unchanged after factory engine service in contrast to the microstructure of the previous industry standard two phase coatings. Further, the growth of a CVD single phase platinum aluminide coating was relatively insignificant compared to two phase coatings during factory engine service. Moreover, the "high temperature low activity" outward grown platinum aluminide coatings were observed to be more ductile than inward grown "low temperature high activity" platinum aluminide coatings. These technical articles indicate only that the outwardly grown, single phase platinum aluminide coatings involved in the factory engine testing program were made by CVD aluminizing the substrate with a prediffused, electroplated platinum layer thereon.